Electrometer amplifier circuits

ABSTRACT

Electrometer amplifier circuits for producing a voltage signal which is proportional to the electrostatic potential on an insulator. In a first embodiment, the electrometer amplifier circuit is controlled such that the output voltage is equal to the sensed electrostatic potential or multiples thereof. In a second embodiment, the circuit output nulls about the surface potential on the insulator.

[451 May 30, 1972 United States Patent Seachman 2,881,266 4/1959Miller................................324/l23X 3,440,525 4/1969Cardeiro............................324/l23X Ned Jay Penfield PrimaryExaminer-Alfred E. Smith [73] Assignee: Xerox Corporation, Rochester,NY.

[22] Filed: Jan. 23, 1970 Attomey-James J. Ralabate, John E. Beck andIrving Keschner ABSTRACT [21] Appl. No.: 5,441

Electrometer amplifier circuits for producing a voltage signal [52] U.S.324/32, 324/123 I which is proportional to the electrostatic potentialon an insu- -.G0lr 27/00, GOlr 5/28, GOlr 1/30 lator. In a firstembodiment, the electrometer amplifier circuit is controlled such thatthe output voltage is equal to the sensed mh m l m d Ld MF 1] .l 8 6 wReferences Cited electrostatic potential or multiples thereof. In asecond embodiment, the circuit output nulls about the surface potentialon the insulator.

UNITED STATES PATENTS 3,448,291 Burk et al..........................328/127 X 12 Claims, 5 Drawing Figures l-IIGHVOLTAGE SUPPLY PATENTEDMY 30 I972 SHEET 10F 4 INVENTOR.

NED J. SEACHMAN BY *M ATTORNEY PATENTEUMM 30 I972 SHEET t [IF 4Xerography, as pertinent to the present invention, comprises an imagereproduction method wherein an electrostatically charged photoconductiveinsulating plate is exposed to a light image and the resultingelectrostatic latent image is developed or made visible through theselective deposition of electrostatically attractable particles. Thelatent image may optionally be transferred or fixed in imageconfiguration to a sheet of paper or other support material.

One of the methods of developing the latent electrostatic image is bymeans of the two component development technique as disclosed by Wise,in U.S. Pat. No. 2,618,552. Two component development is based upon thephenomena of triboelectrification. By rubbing together twotriboelectrically dissimilar materials, an opposite electrostatic chargeis induced in each of the materials. In xerography, finely divided tonerparticles are mixed with relatively coarser carrier beads so that thetoner particles are charged to a polarity opposite that of the latentelectrostatic image. The two component material is then brought intocontact with the exposed xerographic plate where the carrier beads giveup their toner particles to the more highly charged image areas retainedon the plate surface thus making the images visible. The two componentdeveloper material has been used in cascade development systems, asdisclosed by Walkup, US. Pat. No. 2,638,4l6. In conventional cascadedevelopment, the developer material is allowed to flow over an imageretaining plate surface where the image is first developed in the mannerdisclosed by Wise. However, after image development, the toner depletedcarrier beads, still retaining a toner attracting charge, are allowed toclean or scavenge weakly held toner particles from the background ornon-imaged areas on the plate.

A developement technique which produces a high quality image isgenerally characterized in that the xerographic plate, orphotoconductive insulating member, is brought into contact with theelectrostatically attractable particles while spaced adjacent to anequipotential member known as a development electrode. Thisconfiguration causes an electrostatic field to be formed between theplate and the equipotential member in proportion to the charge on theplate and is also effective to increase the electric field above largeareas of uniform charge density. It is these electric fields which causethe electrostatically attractable particles to move to and adhere to theplate for purposes of development. In this way, large solid areas may bedeveloped. However, the potential on the development electrode must beaccurately matched to the minimum potential on the photoconductor ifimages are to be formed with clear backgrounds. Otherwise, thebackground potential produces an electric field between the plate andthe development electrode and the electrostatically attractableparticles are deposited in those areas giving a high background densityin the areas which should be reproduced as white. Xerographicdevelopment is primarily dependent on the potential difference betweenbackground and image voltage, rather than on absolute values, and thebiasing potential placed on the development electrode, is generallymaintained at some level above or below one of these voltages. It hasbeen found that the electrical characteristics of most xerographic platematerials, including the dark discharge rate, will change as the platetemperature changes or with extended plate usage thereby making itextremely difiicult to maintain a uniform quality of development in thistype of system.

A technique for minimizing the background density is to measure theelectrostatic potential on the xerographic plate and adjusting thedevelopment electrode potential to the minimum measured potential. Inorder to measure the electrostatic potential, an electrometer isrequired. The electrometer ideally should be accurate, reliable, simpleand economical. However, the prior art electrometers have certaindeficiencies associated therewith. For example, one of the difficultiesencountered in the use of the prior art electrometers is that theelectrometer contains components having parameters which vary withtemperature and aging. In addition, the capability of adjusting theoutput signal to multiples of the electrostatic potential, as well asbeing equal to it, is lacking. Slow changes in circuit parameters effectthe accuracy of the output potential to electrostatic surface potentialratio of prior an electrometers. Finally, the prior art electrometersutilize a large number of components with a corresponding increase incost.

SUMMARY OF THE INVENTION The present invention provides new electrometeramplifier circuits for measuring the potential of the electrostaticcharge formed on an insulating surface. In the first embodiment. thecircuit includes a probe assembly comprising probe and guard electrodes.The output of the probe electrode is connected to a high input impedancecircuit of approximately unity gain. The output of the high impedancecircuitis connected to a clamping circuit which clamps the signalportion of the output to a stable zero reference level. The output ofthe clamping circuit is coupled to a peak detector via an amplifier. Theoutput of the peak detector is connected to a high voltage amplifier,the output of which is proportional to the electrostatic sur facepotential. The output of the high voltage amplifier is controlled by avoltage divider circuit such that the output voltage is equal to thesensed electrostatic potential or multiples thereof.

In the second embodiment, the high voltage amplifier output is fed backto the guard electrode on the probe assembly and to a high gaindifferential amplifier so that the circuit output nulls about thesurface potential on the insulator.

It is an object of the present invention to provide improved apparatusfor developing a latent electrostatic image.

It is a further object of the present invention to provide improvedelectrometer amplifier circuits for producing a voltage signal which isproportional to the electrostatic potential on an insulator surface, andin particular, for utilization with apparatus for minimizing backgroundon a developed xerographic image. 7

It is still a further object of the present invention to provide novelapparatus for sensing electrostatic potential on an insulating surface.

It is a further object of the present invention to provide novel,economical, simple and accurate electrometer amplifier circuits.

DESCRIPTION or THE DRAWINGS For a better understanding of the inventionas well as other objects and further features thereof, reference is madeto the following detailed description which is to be read in conjunctionwith the accompanying drawings wherein:

FIG. I is a schematic drawing of one embodiment of the novelelectrometer amplifying circuit of the present invention;

FIG. 2is a block diagram of a second embodiment of the presentinvention;

FIG. 3 illustrates schematically a xerographic reproducing apparatusadapted for high speed automatic operation which incorporates the novelcircuitry of the present invention;

FIG. 4 is a front elevation and partial section of the developmentsystem illustrated in FIG. 3 showing the development electrode and thecontrol apparatus which includes the novel apparatus of the present.invention; and

FIG. 5 is a partial side elevation of the sensing probe and shuttermechanism taken along line 5-5 in FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Refen'ing now to FIG. 1, thereis shown a schematic diagram of one embodiment of the present invention.A probe assembly 12, positioned adjacent to an insulating surface 10,such as a photoconductor, generates the input voltage which isproportional to the electrostatic potential on insulating surface 10.The probe'assembly 12 comprises a probe electrode 13 positioned about 0.l 25 inches from insulating surface 10, a metal guard electrode 14 whichsurrounds the probe and which in operation, is maintained at a potentialsubstantially equal to that of the probe electrode 13 to minimizeleakage current, and, a grounded metal member surrounding theprobe-guard assembly to shield the assembly from external electricalfields. The output of the probe assembly 12, which is proportional tothe electrostatic potential on the insulated surface 10, is coupled to ahigh-impedance circuit 16. A high-impedance circuit is required toprevent discharge of the voltage induced on capacitor C1 during theprobe-sense period. As shown schematically in FIG. 1, the high impedancecircuit comprises a MOS (metal-oxide-semiconductor) field effecttransistor Q1 in a source-follower configuration. Q2 is a passive device(i.e., a resistor) to maintain a high input impedance between thegate-to-source terminals of Q1 while also establishing the bias for Q1.Q2 is an N-channel enhancement type MOS field effect transistor whichbiases the N- channel depletion-type MOS field effect transistor Q1 sothat charge carriers are present in the channel with no signal voltageapplied to the gate electrode. A reverse gate voltage is applied to Q2by the potential drop across R1 such that charge carriers in the channelare depleted, thereby reducing the channel conductivity. In thecommon-drain arrangement of Q1, known as a source-follower, the inputimpedance is high, there is no polarity reversal between input andoutput, and the voltage gain is always less than unity, and distortionis low. The input signal is effectively injected between the gate anddrain electrodes, and the output is taken between the source and drain.For a more detailed explanation of MOS field effect transistors,reference may be made to the RCA Transistor Manual, published by RCA,Princeton, New Jersey, 1966, pages 93'through l09. It should be notedthat the Q1, Q2 configuration functions essentially as a vacuum tubecathode follower and may be replaced by'a tube cathode follower or atransistorized emitter-follower stage.

In order to minimize the variation of the quiescent operating voltage of01- with temperature and component aging, a clamping circuit a 18 iscoupled to the output of high impedance circuit 16. The clamping circuit18 clamps the signal portion of the output of circuit 16 to a stablezero reference level and removes the quiescent component from the outputsignal of high impedance circuit 16 which eliminates long term voltagedrift from the electrometer amplifier. The output from the highimpedancecircuit 16 is coupled to the inverting input of amplifier Al viaclamping capacitor C2. The clamping voltage on C2 is maintained by theideal" diode action of Al and CR1. The clamped output signal isconnected to non-inverting gain circuit 20. The non-inverting gaincircuit 20 comprises operational amplifier A2 in a configuration whichachieves a voltage gain which is equal to the value (R4)/(R3) +1, andachieves the necessary impedance transformation between the high outputimpedance of the clamping stage and the low impedance drive required forsubsequent stages. The output of amplifier A2 is coupled to diode peakdetector 22 which comprises semiconductor diode CR2, resistor R6 andcapacitor C3. The signal appearing at the output of amplifier A2, in thenormal operation of the electrometer wherein the electrostatic potentialis periodically sensed, is essentially a series of positive pulses andwhose peak values are proportional to the voltage on surface 10.Capacitor C3 is charged by the positive pulses to the peak value thereofand provides a substantially constant output voltage equal to the peakinput voltage. Re-

sistor R6 provides a discharge path for capacitor C3 to permit the peakdetector to follow slow decreases in the peak value of the input pulses.The output across capacitor C3 is connected to the inverting input ofdifferential amplifier 24. The output of differential amplifier 24 iscoupled to the base electrode of transistor 03 via the parallelconnection of resistor R7 and capacitor C4. The base electrode oftransistor Q3 is shunted to ground via semiconductor diode CR3.Differential amplifier 24 and transistor 03 act essentially as a linearhigh voltage amplifier. The output voltage of amplifier 24 is applied tothe base of transistor Q3 and an inverted, amplified signal E0 isproduced at the collector of transistor Q3. The output E0 of transistorQ3 is coupled back to the other input of differential amplifier 24 via afeedback network comprising variable potentiometer 30. The variablepotentiometer 30 comprises ages at the input of the differentialamplifier are equal. The

output voltage E0 will thus always be a fixed multiple of the voltage onC3 determined by the setting of tap 32. The multiple may be selectedsuch that the output E0 is equal to the sensed photoconductorelectrostatic potential;

High voltage supply 34, with an output of approximately 650 volts,biases the collector circuit of transistor Q3 and provides the necessarysupply voltage across potentiometer 30. The output voltage E0 variesfrom approximately zero volts (when O3 is saturated) to approximately500 volts (when Q3 is operating close to cut-off)..

A more detailed description of the operation of differential amplifier24, transistor Q3 and potentiometer 30 follows. If it is assumed thatthe voltage across capacitor C3, Vc3, is 50 volts and tap 32 has beenset so that the voltage thereacross, V32, is 0.5 Eo which is greaterthan the voltage across C3, the positive difference in voltage at theinput of differential ampli fier 24 is transmitted to the base oftransistor Q3. Transistor Q3 is caused to conduct to a degree dependenton the magnitude of the positive voltage applied to its base. A greaterportion of the current from high voltage supply 34 is directed to thecollector of Q3, thereby decreasing the voltage across tap 32 so thatthe inputs to the differential amplifier 24 are equal. Therefore Sinceit has been assumed that Vc3 50 volts,

Eo=2 50= volts.

It should be noted that useful output signals may be obtained at theoutput of amplifier A2 and across capacitor C3.

Referring now to FIG. 2, a'block diagram of the second embodiment of thepresent invention is illustrated. For illustrative purposes, theelectrostatic potential is shown deposited on a photoconductive drum 40,such as that utilized in xerography. The probe electrode 42 is coupledto the input of a high input impedance, unity gain amplifier 44. As setforth in reference to FIG. I, amplifier 44 may comprise a cathodefollower. The output of amplifier 44 is coupled to one input of a highgain differential amplifier 46. Amplifier 44 maintains a sufiicientlyhigh input impedance to allow the remainder of the circuit to respondbefore capacitor C1 is discharged. It should be noted that Cl generallycomprises stray capacitance and the input capcitance of amplifier 44.Capacitance C2 represents the stray capacitance between guard electrode43 and ground. The ratio of C2 to C1 is generally about 100. If theratio is significantly smaller, a physical capacitor may be added to theinput of amplifier 44. The small positive output voltage from amplifier44 is applied to one input of high gain differential amplifier 46 whereit is compared with the voltage on the guard electrode line. Initially,a higher positive voltage will be induced on the probe electrode 42 thanon guard electrode 43 because of the capacitance relationship betweencapacitor C1 and C2. The output of differential amplifier 46 thereforewill initially be positive and will cause transistor O4 to conduct. Thecurrent through transistor Q1 causes a voltage drop across resistor R1and the guard and output voltages begin to rise. As the guard voltageincreases, the probe voltages will also continue to increase until theprobe voltage matches the potential on the photoconductive surface. Theguard voltage and thus the output voltage Ea will, at this point, bevery nearly equal to the probe voltage. if the guard voltage tries toincrease beyond that of the probe voltage, the output of differentialamplifier 46 will go negative, reducing the current to Q4, therebyreducing the voltage across resistor R1. Should the guard voltage dropsignificantly below the probe voltage, the output of the differentialamplifier will be more positive, increasing the current in transistor Q4and thus increasing the guard voltage. A stable condition is thereforereached when the probe voltage and guard voltage are nearly equal to theelectrostatic surface potential on the photoconductor. A diode peakdetector for providing a relatively constant output voltage equal to thepeak input voltage is provided at the emitter electrode of transistorQ4. The diode peak detector comprises semiconductor diode CR1, resistorCR2 and capacitor C3.

The above circuit has several advantages. Since it tends to null aboutthe electrostatic potential on the photoconductor surface, slow changesin circuit parameters will have negligible effect on the accuracy of theoutput voltage to surface potential ratio. ln addition, the probeelectrode and associated circuitry float above ground at thephotoconductor surface potential. The circuit uses a limited number ofcomponents, therefore its cost is correspondingly low.

FIGS. 3, 4 and 5 illustrate apparatus in which the novel circuitry ofthe present invention may be utilized. in particular, a reproducingapparatus comprising a xerographic plate including a photoconductivelayer of a light sensitive material placed on a conductive backing andformed in the shape of a drum generally designated 40, is shown. Thedrum is journaled for rotation in the machine frame (not shown) upon ahorizontal support shaft 42. The xerographic drum is rotated in thedirection indicated in FIG. 3 to cause the photoconductive surface topass sequentially through a plurality of xerographic processingstations.

For the purpose of the present disclosure, the several xerographicprocessing stations in the path of movement of the drum surface may bedescribed functionally as follows:

A charging station A, in which a uniform electrostatic charge isdeposited on the moving photoconductive surface;

An exposure station B, wherein the light image or radiation pattern ofan original document to be reproduced is projected on to the drumsurface to dissipate the charge found thereon in the light exposed areasso as to form a latent electrostatic image which is retained thereon;

A developing station C, at which a two component xerographic developingmaterial having toner particles possessing an electrostatic chargeopposite to the image charge found on the drum surface are cascaded overthe upwardly moving drum surface whereby the charged toner particlesadhere to the electrostatic latent image areas making the images visiblein the configuration of the original to be reproduced;

A transfer station D, in which the xerographic powder image iselectrostatically transferred from the drum surface to a final supportmaterial; and

a drum cleaning and toner collecting station E, where the drum surfaceis first treated with a corona discharge to neutralize any residualcharge found thereon and then cleaned with a flexible cleaning blade toremove residual toner from the drum surface. A reservoir for collectingand storing the removed residual toner and an incandescent panel toaffect substantially complete the discharge of any residualelectrostatic image remaining thereon is also included.

The charging station is preferably located at the bottom of the drum inthe position indicated by reference A shown in FIG. 3. The chargingarrangement consists of a corona charging device 43 which includes acorona discharge array of one or more corona discharge electrodes thatextend transversely across the drum surface and are energized by a highpotential source. The corona discharge device is substantially enclosedwithin a shielding member and is adapted to generate a positive chargeconfined'within this specific area.

Next subsequent thereto in the path of drum rotation is an exposurestation B wherein a flowing light image of a stationary original isplaced on the drum surface. Basically, the exposure station comprises anoptical scanning and projecting assembly and a stationary transparentcopyboard 44 adapted to support the original to be reproduced. A movinglight source 45 is mounted below the copyboard and is arranged to movein timed relation with a lens element 48 to scan the original supportedupon the copyboard thus creating a flowing light image of the original.The light image is projected by the lens through a folded opticalsystem, including an object mirror 49 and an image mirror 50, arrangedto focus the light image on the bottom of the drum.

Positioned adjacent to the exposure station is a developing station C inwhich is positioned a developer housing 52 having a reservoir areatherein capable of supporting a quantity of two component developermaterial including negatively charged toner particles. A bucket-typeconveyor 53 transports developer material from the lower reservoir areato the upper part of the developer housing where it is deposited inentrance chute 51. Any suitable drive means can be used to rotate thebucket conveyor in the direction indicated. As will be explained ingreater detail below, the developer material moves downwardly in contactwith the upwardly moving photoconductive drum surface through acompletely electroded development zone wherein the latent electrostaticimage on the drum surface is developed. The unused developer materialpasses from the development zone and is directed back into the reservoirarea by means of a pick-off bafile. A toner container and dispensingapparatus 56 is affixed to the developer housing and is adapted to addfresh toner material into the reservoir area in proportion to the amountof toner deposited on the drum surface.

An image transfer station D is positioned adjacent to the developingstation. Individual sheets of final support material are fed seriatiminto the sheet registering and forwarding apparatus, generallyreferenced by numeral 57, from either of two supply trays 66 and 67. Theindividual sheets are properly registered and then forwarded into movingcontact with the rotating drum surface and the developed electrostaticimage transferred from the drum to the final support material by meansof a transfer corotron 55. In operation, the electrostatic field createdby corona discharge device electrostatically tacks or bonds the transfermaterial to the drum surface wherein the transfer material is caused tomove in synchronous relation with the rotating drum surface.

A mechanical stripper finger 58 is pivotally mounted in close proximityto the drum surface immediately downstream from the transfer station.The stripper finger is arranged to move between the copy sheet and thedrum surface breaking the electrostatic bond holding the sheet to thedrum and to direct the support material into moving contact with thebottom surface of a stationary vacuum transport 59.

A combination of heat and pressure energy is employed in the presentapparatus to fix the xerographic image to the final support material.The image bearing support material is guided into the fusing assembly 63as it is moved along the bottom surface of transport 59. Fuser assembly63 comprises an upper fuser roll 64 and a lower fuser roll 65 arrangedto coact to deliver a pressure driving force to a sheet introducedtherebetween. A radiant heat source 68 is positioned transverse to thelower fuser roll and applies heat energy to the surface of the roll. Theroll, which is specially coated, stores the heat energy on its surface.As the rolls are rotated in the, direction indicated, both heat energyand pressure energy are delivered by the roll into the imaged areasthereby fixing the image to the final support material. After leavingthe fuser assembly, the now fixed copies are transported through acircular paper path, as illustrated in FIG. 3, into a catch tray 69where the copy can be conveniently collected by the machine operator.

Referring now to FIG. 4, the two component developer material is firsttransported from the reservoir or storage area in developer housing 52and deposited in a hopper-like input chute 51 by means of a bucketconveyor system 53. A quantity of developer material is stored withinthe input chute and flows downwardly through a constrained opening 71into the introductory region of development zone 70. The front wall ofthe development zone is formed by the movable drum surface 40 while therear wall is formed by a series of downwardly extended electrodesrunning transversely across the photoconductive coating on the drumsurface. The electrodes are supported in spaced parallel relation to thedrum surface by means of an insulating support frame 73 secured to thewalls of the developer housing by any suitable means. The individualelectrodes are separated from each other by dielectric blocks 72 so thatthe rear wall of the development zone presents a substantiallycontinuous surface to the developer material introduced therein.Although not shown, end seals are provided between the electrodes andthe drum surface to substantially enclose the development zone thusproviding a conduit through which the developer material gravity flows.The development zone extends from the introductory opening 71 oppositetothe upper drum surface to a point well below the horizontal centerline of the drum.

Basically, the control electrodes are biased so that the developermaterial perfonns a cleaning function in the upper development zonewhile a preponderance of image development takes place in the lowerinverted development zone region thereof. By varying the chargepotential and magnitude on the various electrodes, the concentration andpositioning of toner in the flow stream can be controlled to regulatethe degree of development and cleaning obtained in each of theelectroded regions.

The first electroded region through which a latent electrostatic imageis transported is the region influenced by a low potential electrode 75physically located in the bottom of the development zone 70. The termlow potential, as herein used, refers to a potential which is lower thanthe background potential on the xerographic plate surface. This term isbroad enough to include a grounded electrode or even a floatingelectrode. Because of the unique control features of the presentdeveloping apparatus, carrier beads which are properly toned for optimumdevelopment are flowing through this lower development zone. In thispreferred embodiment, the lowjpotential electrode is placed at a groundpotential so that an extremely strong force field is established tendingto force the negatively charged toner particles toward the plate side ofthe development zone. At the same time, the electrode acts as aconventional development electrode to enhance the latent electrostaticforce fields, particularly the force field associated with solid imagedareas, so that extremely rapid and efficient image development isproduced in this region.

The leading edge of the low potential electrode, that is, the edge thatfirst presents itself to the developer flow, is chamfered to direct thedeveloper flow upwardly into contact with the drum surface. In thismanner, toner particles are both physically dislodged from the carrierbeads and transported into contact with the plate surface. The airbornetoner particles, because they are in a free state, are readily attractedinto th'eimage areas so that extremely rapid development takes place inthis region. Overdevelopment of the xerographic plate, in fact, mayresult. However, as will be explained below, an overdeveloped conditionin this region can be tolerated by the present development system.

The next electrode positioned in the direction of drum rotation is themain development electrode 76. The main development electrode is biasedat a potential somewhere between the image potential and the backgroundpotential found on the plate surface and preferably at somepredetermined level above the background voltage. When an imaged area onthe drum surface is transported through the main development electroderegion, the force field associated with the imaged area, being of ahigher magnitude than the electrode force field, predominates. The tonerin the flow stream adjacent to the imaged surface is thus attracted intothe imaged areas. However, when a non-imaged or background area is movedthrough the main developing region, the electrode force field dominatesand the toner particles are pulled away from the plate surface towardsthe backside of the development zone. The developer material moving incontact with the nonimaged drum surface therefore tends to mechanicallyscrub the background areas to dislodge randomly dispersed,'weakly held,toner particles from the plate. This dislodged toner, coming under theinfluence of the stronger electrode force field, is similarly attractedtowards the electroded side of the system. As can be seen, the maindevelopment electrode, in effect, acts as a self-regulating device toeither complete image development or to clean up background areas inthis region.

The now xerographically developed photoconductive surface next movesinto the last development region in which an extremely strong tonerattracting force field is produced by a clean-up electrode 77. A biasingsource 74 is electrically connected to the clean-up electrode andelectrically biases electrode at a potential greater than the imagepotential on the plate surface, preferably 300 volts abovethe imagepotential. The bias potential is sufficiently high enough to attract anextremely heavy concentration of toner in the flow stream to thebackside of he development zone. The carrier beads moving in contactwith the plate surface become toner depleted and therefore are capableof both mechanically scrubbing and electrostatically scavenging unwantedbackground development from the plate surface. Here again, the strongelectrode force field attracts random toner particles from the vicinityof the plate surface so that a clearwell-defined developed xerographicimage leaves the development zone. Clean-up electrode 77 is turned at aslight radius at the developer entrance 41 and extends outwardly andupwardly from the development zone to form the bottom wall of the inputchute 51. The opposite wall of the input chute is formed by anelectrically isolated baffle 78 secured to the developer housing wall bysuitable means. The lower end of the bafilehas a lip formed thereoncomplementary to the tuming radius of the clean-up electrode so that auniform opening 71 is provided through which the developer materialenters the development zone in a relatively undisturbed flow. Bafile 78is placed at a ground or toner repelling potential which, when combinedwith the toner attracting force field of electrode 77, forces apreponderance of the toner particles in the flow to the backside of thesystem. Because of the physical configuration of the input chute and thestrong electrostatic force field associated therewith, the formation oftoner powder clouds in and about the introductory region to thexerographic development zone is minimized thus preventing unwantedbackground develop ment from occurring. A strong toner concentration isthus established on the backside of the flow stream prior to thedeveloper material entering the development zone so that toner depletedbeads initially contact the drum surface as it leaves the developmentzone.

The electrostatic properties of many known photoconductive plates tendto change slightly with changes in temperature or with extended plateusage. This change or drifting" in the electrical plate parameters haslittle or no effect on the control features of the low potentialelectrode or the clean-up electrode. However, this is not the case inregard to the main development electrode. As noted, the main developmentelectrode is held at some predetermined voltage between plate imagevoltage and plate background voltage, and preferably at some fixedvoltage above the plate background voltage. Here the difference betweenthe background,or reference, voltage and the desired electrode voltageissmall and any electrical drifting in the plate voltage will normally bereflected in a change in the quality of the development produced.

The circuitry as described with reference to FIGS. 1 and 2 hereinabovemay be utilized to regulate the bias potential on the main developmentelectrode in order to compensate for changes in the plate of voltage sothat images of uniform quality are produced by the present developingapparatus. Physically, the main development electrode control systemcomprises: a sensing probe assembly adapted to periodically sample thelevel of background voltage on the rotating drum surface; a signalgenerating device adapted to convert the sampled voltage into acontinuous control signal and a fixed or adjustable power supplyresponsive to the control signal wherein the development electrode ismaintained at a predetermined voltage level in regard to the sampledplate voltage.

. A sensing probe support housing 79 (FIG. is secured in the machineframe and is positioned between the xerographic exposure station and thedeveloping station. The sensing probe assembly 80 is seated within thesupport housing in juxtaposition to one end of the drum surface and isarranged to sense a narrow sample strip on the photoconductive surfacenear the edge of the drum.

The sample strip is passed through the charging and exposing stationsand is placed at the plate background potential. The strip issufficiently offset to one side of the drum surface so that its presencedoes not interfere with the normal machine operations.

A solenoid actuated shutter 83 is slideably mounted within the guidesprovided in the upper portion of the support housing. The shutter isoperatively connected to a solenoid SOL-1 by means of a crank arm 84.The crank arm is rotatably mounted upon a pivot pin 85 and the pinsecured in the body of the housing. The lower end of the crank arm ispivotally affixed to the solenoid actuator arm 86 while the opposite endof the arm is similarly connected to a downwardly turned dependentflange 88 formed in the lower part of shutter 83. A pin 87, passingthrough the upper part of the crank arm, rides in a vertically alignedslotted hold (not shown) formed in flange 88 which permits the shutterto move in a horizontal direction as the crank arm is rotated. Inoperation, the solenoid is energized once during each copying cycle.Energization of the .solenoid pulls actuator arm 86 upwardly causing thecrank arm to rotate in a counterclockwise direction. As the crankrotates, the shutter is moved back exposing the sensing probe to thesampling strip.

The machine logic system, generally referenced 90 in FIG. 4, is arrangedto generate a mid-scan trigger signal during each xerographic copyingcycle. In practice, the signal which energizes solenoid SOL-l, isgenerated as the scanning lens 48 physically passes the midpoint of itsprogrammed path of travel. The trigger signal is passed to the novelelectrometer circuitry 92 of the present invention. A voltage indicativeof the plate background voltage is sensed by the probe assembly andamplifier circuits of the present invention and a continuous outputcontrol signal is generated which is applied to a fixed or adjustablepower supply 94. The power supply is operatively connected to the maindevelopment electrode 76 and regulates the electrode potential at apredetermined level above the background voltage on the plate. Thedetails of the machine logic system 90 are not set forth since thepresent invention is concerned with novel circuitry for generating anelectrical signal which is proportional to the electrostatic charge orbackground voltage on the sampling strip. The description of FIGS. 3 to5 is set forth to illustrate the type of apparatus in which thecircuitry of the present invention may be utilized. It should be notedthat the novel electrometer amplifier circuits of the present inventionmay be utilized in any system wherein it is desired to produce a voltageproportional to the electrostatic potential formed on an insulatingsurface.

What is claimed is:

1. Apparatus for generating a modified electrical signal proportional tothe electrostatic potential formed on an insulating surface comprising:

a. means positioned adjacent to said insulating surface for producing anelectrical signal proportional to said electrostatic potential,

b. first circuit means connected to said producing means, said firstcircuit means having a high input impedance and a low gain,

c. means connected to said first circuit means for providing a stablereference level for the output of said first circuit means, d. secondcircuit means connected to the output of said reference level providingmeans, said second circuit means providing a controlled voltage gain andimpedance transformation between the reference level means andsubsequent apparatus, e. a peak detector connected to the output of saidsecond circuit means, f. means for generating a signal which isproportional to the difference between the signals applied to its inputterminals, means for connecting the output of said peak detector to oneinput terminal of said generating means,

h. means for amplifying the output of said generating means, the outputof said amplifying means being said modified electrical signal, and

i. means for coupling the output of said amplifying means to the otherinput of said generating means.

2. The apparatus as defined in claim 1 wherein said generating means isa differential amplifier.

3. The apparatus as defined in claim 2 wherein said coupling meanscomprises a variable potentiometer having an adjustable tap, the tap ofsaid potentiometer being connected to the other input of saiddifferential amplifier.

4. The apparatus as defined in claim 3 wherein the magnitude of saidmodified electrical signal is determined by the position of saidadjustable tap.

5. The apparatus as defined in claim 4 wherein said modified electricalsignal equals said electrostatic potential.

6. The apparatus as defined in claim 4 wherein said modified electricalsignal is a fixed multiple of said electrostatic potential 7. Theapparatus as defined in claim 1 wherein said insulating surfacecomprises a photoconductor.

8. Apparatus for generating an electrical signal which is proportionalto the electrostatic potential formed on an insulating surfacecomprising:

a. a probe assembly positioned adjacent said insulating surface, saidprobe assembly comprising a probe electrode and a guard electrode, theprobe electrode producing an electrical signal proportional to saidelectrostatic potential,

b. means for producing a signal which is proportional to the differencein magnitude between signals applied to its input terminals,

c. first means for coupling the electrical signal appearing at theoutput of said probe electrode to one input of said producing means,

d. amplifier means connected to the output of said producing means foramplifying the output thereof, the output of said amplifier means beingproportional to the electrostatic potential on said insulating surface,and

e. second means for coupling the output of said amplifier means to saidguard electrode and to the other input terminal of said producing means,whereby the output of said amplifier means is nulled about saidelectrostatic potential.

9. The apparatus as defined in claim 8 further including a peakdetecting means connected to the output of said amplifier means, theoutput of said peak detecting means being a voltage proportional to thepeak value of the output of said amplifying means.

10. The apparatus as defined in claim 9 wherein said first couplingmeans comprises an amplifier having a high input impedance andsubstantially unity gain.

11. The apparatus as defined in claim 10 wherein said producing meanscomprises a differential amplifier.

12. The apparatus defined in claim 8 wherein said insulating surfacecomprises a photoconductor.

1. Apparatus for generating a modified electrical signal proportional tothe electrostatic potential formed on an insulating surface comprising:a. means positioned adjacent to said insulating surface for producing anelectrical signal proportional to said electrostatic potential, b. firstcircuit means connected to said producing means, said first circuitmeans having a high input impedance and a low gain, c. means connectedto said first circuit means for providing a stable reference level forthe output of said first circuit means, d. second circuit meansconnected to the output of said reference level providing means, saidsecond circuit means providing a controlled voltage gain and impedancetransformation between the reference level means and subsequentapparatus, e. a peak detector connected to the output of said secondcircuit means, f. means for generating a signal which is proportional tothe difference between the signals applied to its input terminals, g.means for connecting the output of said peak detector to one inputterminal of said generating meaNs, h. means for amplifying the output ofsaid generating means, the output of said amplifying means being saidmodified electrical signal, and i. means for coupling the output of saidamplifying means to the other input of said generating means.
 2. Theapparatus as defined in claim 1 wherein said generating means is adifferential amplifier.
 3. The apparatus as defined in claim 2 whereinsaid coupling means comprises a variable potentiometer having anadjustable tap, the tap of said potentiometer being connected to theother input of said differential amplifier.
 4. The apparatus as definedin claim 3 wherein the magnitude of said modified electrical signal isdetermined by the position of said adjustable tap.
 5. The apparatus asdefined in claim 4 wherein said modified electrical signal equals saidelectrostatic potential.
 6. The apparatus as defined in claim 4 whereinsaid modified electrical signal is a fixed multiple of saidelectrostatic potential .
 7. The apparatus as defined in claim 1 whereinsaid insulating surface comprises a photoconductor.
 8. Apparatus forgenerating an electrical signal which is proportional to theelectrostatic potential formed on an insulating surface comprising: a. aprobe assembly positioned adjacent said insulating surface, said probeassembly comprising a probe electrode and a guard electrode, the probeelectrode producing an electrical signal proportional to saidelectrostatic potential, b. means for producing a signal which isproportional to the difference in magnitude between signals applied toits input terminals, c. first means for coupling the electrical signalappearing at the output of said probe electrode to one input of saidproducing means, d. amplifier means connected to the output of saidproducing means for amplifying the output thereof, the output of saidamplifier means being proportional to the electrostatic potential onsaid insulating surface, and e. second means for coupling the output ofsaid amplifier means to said guard electrode and to the other inputterminal of said producing means, whereby the output of said amplifiermeans is nulled about said electrostatic potential.
 9. The apparatus asdefined in claim 8 further including a peak detecting means connected tothe output of said amplifier means, the output of said peak detectingmeans being a voltage proportional to the peak value of the output ofsaid amplifying means.
 10. The apparatus as defined in claim 9 whereinsaid first coupling means comprises an amplifier having a high inputimpedance and substantially unity gain.
 11. The apparatus as defined inclaim 10 wherein said producing means comprises a differentialamplifier.
 12. The apparatus defined in claim 8 wherein said insulatingsurface comprises a photoconductor.